Although many in Congress see the war in Afghanistan as too costly, "the cost of failure" is high and decisions should not be made using short-term thinking, Defense Secretary Robert Gates told ABC News' Diane Sawyer during an exclusive interview in Kabul, Afghanistan.

"We've invested a huge amount of money here. We've invested 1,254 lives up to this point. So what's the cost of getting it wrong," said Gates. "Congress is almost always impatient. I remember in the spring of 2007, people saying this war is lost in Iraq."

In Kandahar, Gates told troops he's confident the United States will "strike the right balance" on the withdrawal of U.S. forces from Afghanistan.

"Nobody wants to give up the gains that have been won at such a hard cost, and nobody wants to give our allies the excuse to run for the exits," Gates told the troops at Forward Operating Base Walton in Kandahar.

Gates said it was a "no-brainer" that combat troops should be the last to leave as he thanked them for their service.

"For my money ... if it were up to me, I would leave the shooters for last," he said.

His trip comes as he evaluates the situation on the ground in Afghanistan, where the US effort costs $10 billion per month, before he retires at the end of June.

"I feel your hardship and your sacrifice and your burden, and that of your families, more than you can possibly know. You are, I believe, the best our country has to offer, and you will be in my thoughts and prayers, every day for the rest of my life. Thank you," Gates told troops Sunday while choking back tears.

Given the troop numbers, however, Gates said the drawdown this summer would likely be a combination of combat and support elements.

The actual timeline of the troop drawdown has yet to be decided, and the defense secretary said Gen. David Petraeus, the current commander of the forces in Afghanistan, and Lt. Gen. John Allen, his successor, will play key roles in deciding that.

"I think the mix of what comes out is really up to Gen. Petraeus and then Gen. Allen in terms of composition and which units are selected," Gates said.

Drones Crucial in War against Terror

While Gates thanked the troops, he also acknowledged that drones have proven to be crucial in the war against terror.

"First of all it has to be acknowledged that these drones have a significant role in taking a lot of Taliban leaders, trainers off the table. The question really then becomes the role of the drones, our relationship with Pakistan and how this all fits together," Gates told "World News" anchor Diane Sawyer.

Gates will be joined by Petraeus, the outgoing commander of the International Security Assistance Force in Afghanistan (ISAF), Monday for their first joint interview with Sawyer.

The two leaders met on Saturday after Gates arrived in Kabul.

On Saturday, a U.S. missile strike reportedly killed terrorist Ilyas Kashmiri, a contender poised to take over al Qaeda.

Both American and Pakistani officials have said they have not been able to confirm Kasmiri's death.

The report comes as both countries work to on easing tension in the wake of Osama bin Laden's death last month.

"We've gone through a difficult spell with the Pakistani government. The reality is that we need each other. And so working our way through that is a complicated business. It is a complicated relationship. And clearly the drones are a piece of that," Gates told Sawyer.